<canvas id="canvas" width="100" height="100"></canvas>
<script>
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
ctx.globalAlpha = 0.5;
ctx.fillStyle = '#000';
ctx.fillRect(25, 45, 50, 20);
ctx.fillStyle = '#0f0';
ctx.fillRect(25, 25, 50, 20);
</script>